difference between max(datecolumn) and max(to_date(datecolumn),'dd-mm-rrrr')

[DaPi]

My guess is that your data includes two dates:
31-MAY-2003 and 03-JUL-1903

MAX(datecolumn) gives the correct value: 31-MAY-2003.

What I think is happening in the second expression is:
For the TO_DATE to work, datecolumn is somehow first converted to characters (it expects character input NOT a date - I'm surprised it does not give an error):
03-JUL-03
This then converted back to date with -RRRR becomes:
03-JUL-2003
which is bigger than 31-MAY-2003

(All this can be avoided if you make sure your default date format includes YYYY).

. . on the other hand I could be wrong.